Control for basic oxygen steelmaking furnace

ABSTRACT

Method for controlling the refining of a molten bath of steel in a basic oxygen vessel by the steps of continually measuring the intensity of the flame at the mouth of the vessel and the flow rate of oxygen into the vessel, determining from a consideration of flame intensity drop-off a bench mark signaling that the oxygen blow and carbon removal processes are in the final stages, computing the amount of carbon remaining in the bath and the carbon removal rate at the bench mark, and thereafter periodically computing the amount of carbon left in the bath until computed carbon left matches the desired carbon content in the refined steel, whereupon the oxygen blow is terminated.

United States Patent 1191 Roessing [54] CONTROL FOR BASIC OXYGENSTEELMAKING FURNACE [75] Inventor: Keith W. Roessing, Pittsburgh, Pa.

[73] Assignee: Allegheny Ludlum Industries, Inc., Brackenridge, Pa.

[22] Filed: July 22, 1970 [21] Appl. No.: 57,090

[52] US. Cl. ..75/60, 75/59, 235/15 1.12 [51] Int. Cl. ..C2lc 7/04, C2lc7/06' [58] Field of Search ..75/59, 60

[56] References Cited UNITED STATES PATENTS 3,607,230 9/1971 Wintrell..75/60 2,807,537 9/1957 Murphy.... ..75/60 2,207,309 7/1940 Work..75/60 3,485,619 12/1969 Maatsch et al. ..75/60 3,372,023 3/1968Krainer et al... ..75/60 2,801,161 7 1957 Murphy ..75/60 2,354,4007/1944 Percy ..75/60 FOREIGN PATENTS OR APPLICATIONS 125,264 1960U.S.S.R ..75/60 1 March 6, 1973 610,018 12/1960 Canada ..75/60 PrimaryExaminer-L. Dewayne Rutledge Assistant Examiner.l. E. LegruAttorneyBrown, Murray, Flick & Peckham s7 ABSTRACT Method forcontrolling the refining of a molten bath of steel in a basic oxygenvessel by the steps of continually measuring the intensity of the flameat the mouth of the vessel and the flow rate of oxygen into the vessel,determining from a consideration of flame intensity drop-off a benchmark signaling that the oxygen blow and carbon removal processes are inthe final stages, computing the amount of carbon remaining in the bathand the carbon removal rate at the bench mark, and thereafterperiodically computing the amount of carbon left in the bath untilcomputed carbon left matches the desired carbon content in the minated.

6 Claims 3 Drawing Figures CONTROL FOR BASIC OXYGEN STEELMAKING FURNACEBACKGROUND OF THE INVENTION As is known, the primary purpose of a basicoxygen steelm aking furnace operation is the elimination of carbon fromthe scrap and hot metal charged to thereby quickly and economicallyproduce a mass of molten steel which may be alloyed and made into aspecific product. Operational practices which increase the time requiredto produce a heat of steel or which reduce the overall yield from thefurnace are detrimental to the economics of the operation. Operationalpractices which reduce heat time or increase yield are economicallybeneficial. One important consideration, particularly in a plantproducing low carbon steels, is the determination of end-point carbon.If the blowing operation is discontinued prematurely (i.e., the endpointcarbon level is higher than required) it is necessary to reblow theheat. In addition to extending the heat time, the reblow operation hasbeen observed to reduce yields and influence certain quality factors.Overblowing the heat (i.e., discontinuing the blow at a carbon levelsignificantly below the desired level) results in excessive oxidation ofother materials and reduces yield. It is highly desirable, therefore, toprovide a method which will respond to the dynamics of the basic oxygenfurnace operation and predict the carbon content of the bath during thefinal seconds of the blow, in order that the blow can be stopped at thedesired carbon end point.

SUMMARY OF THE INVENTION In accordance with the present invention,end-point carbon is determined primarily from a consideration of theintensity of the flame at the mouth of the basic oxygen vessel, togetherwith the flow rate of oxygen into the vessel and the total oxygenconsumed at the point of bench mark recognition. The bench mark isdefined as the point at which flame intensity with respect to time dropsalong a predetermined slope. Bench mark recognition is achieved byperiodically sampling a signal proportional to flame intensity andcomparing the samples with stored empirical data in a computer inaccordance with pattern recognition techniques. Once the bench mark isdetected, the computer computes the initial estimate of carbon in thebath in accordance with the equation:

where:

FL is the value of the flame intensity at the point of bench markrecognition; C is the total oxygen consumed from the beginning of theblow to the point of bench mark recogni-' tion; k and k are coefflcientsbased on empirical data.

At the same time, (i.e., at the point of bench mark r recognition) thebase carbon removal rate is determined in accordance with the equation:

where:

k;, is an empirically determined coefficient.

Having established these base conditions with the equations given above,a continuous prediction, once each second, is estimated by the computerfrom the relation:

t r-r 1 where:

C,- is the instantaneous value of predicted carbon in the bath;

C is the previous value of predicted carbon in the bath taken one secondbefore; and AC,- is the difference between C,- and C AC,-, in turn, isdetermined once each second from the equation:

where:

FL is the current flow intensity as measured by the smoothed signal;

FL is the flame intensity associated with the point of flame intensityversus time. Since vessel additions may be made prior to the time ofbench mark recognition which would give a false indication of theoccurrence of a bench mark, means are provided to prevent initiation ofthe program for determining bench mark recognition until a short timebefore the end of the blow when a true bench mark recognition can occur.Usually this means comprises apparatus for determining total oxygenflow, the arrangement being such that if total oxygen flow has notexceeded a predetermined minimum, it is known that a true bench markcannot occur since the refining process has not progressed far enoughfor this condition to occur.

Either a running indication of carbon content can be displayed to anoperator such that he can terminate the blow when the indicated carboncontent reaches that specified for the steel being blown, or the blowcan be terminated automatically when the desired carbon content isreached.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic diagram illustrating the process of the invention;and

FIGS. 2A and 2B are illustrations of flame intensity versus time beforeand after filtering, respectively, and showing the manner in which abench mark is recognized.

With reference now to the drawings, and particularly to FIG. 1, a basicoxygen converter 10 is shown comprising an outer steel shell 12 providedwith an inner refractory lining l4 and having an upper open mouth 16.Extending through the mouth 16 is a water-cooled lance 18 having a waterinlet pipe 20, an interior cooling passage 22 and a water outlet pipe24. The lance 18 is also connected to suitable lance-raising means, not

shown.

The vessel contains a bath 26 of molten metal, above which there may bea layer 28 of slag. The vessel 10 is adapted to be turned on trunnions30 by conventional means, not shown. This permits the discharge of thefinished steel through the mouth 16 or, if desired, through a suitabletap hole in the side of the vessel. Oxygen is fed from a supply means 32through a line 34 containing a value 36 such that when the refining isin progress, the bath 26 is impinged upon, as at 38, with the jet ofoxygen issuing from the lance 18 penetrating the bath 26 more or lessdeeply, depending upon the position of the lance and the velocity of theoxygen jet. Those skilled in the art will understand that the apparatusthus far described is conventional.

A flame intensity sensing means 40 is trained upon an area of the sideof the lance 18. Since the lance is water-cooled, this area of the lancewill be essentially black and will emit no radiation. This is necessaryto prevent radiation from the hot walls of the vessel, for example, fromaffecting flame intensity readings The flame intensity sensing means 40may comprise any one of a number of suitable devices, as will beunderstood by those skilled in the art. For example, a bolometer mightbe used. It is preferred, however, to use a photovoltaic energyconverter, and in particular, a semiconductor photovoltaic energyconverter or solar cell. Satisfactory results have been obtained with aLand-type 000 35-50-48 Silicon Solar Cell with a 0.750 inch aperture.Although various physical arrangements are possible, satisfactoryresults have been obtained with such a cell, enclosed in a 2-foot pipewhich serves to protect the lens of the cell from a buildup of dustparticles, the pipe being attached to a swivel mount and this entireassembly being covered with a steel cage, in order to prevent accidentalmovement of the cell. The signals produced by the flame intensitysensing means 40 are filtered in a filter circuit 42 and applied to adigital computer, enclosed by broken lines and identified generally bythe reference numeral 44. Other inputs to the digital computer 44include a signal on lead 46 derived'frorn a flow meter 48 proportionalto the oxygen flow rate through the lance 18. The third input to thecomputer 44 is pulses from pulse generator 50 on lead 52, the number ofwhich is indicative of the total oxygen consumed during a blow.

With reference now to FIG. 2A, the output signal derived from theintensity detector 40 during a typical oxygen blow is shown. The graphsof FIGS. 2A and 28 cover only the latter part of an oxygen blow, aboutseven minutes. The total blow takes about to minutes. Note that at thebeginning of the last 7-minute period, the flame intensity is relativelyhigh; however as the blow continues and the carbon and other impuritiesare burned out of the steel, the intensity gradually diminishes. In FIG.2B, the output of the filter circuit 42 is shown. Note that betweentimes t, and t the curve 54 moves along a plateau. However, at time itbegins to move downwardly with decreasing flame intensity along a moreor less constant slope. This is utilized in accordance with theinvention in order to determine a bench mark 56, at which point aninitial carbon prediction is made and carbon predictions are thereaftermade at l-second intervals until the termination of the blow. it isduring the period between time t shown in FIG. 2B and the end of theblow that most of the carbon is burned out of the molten metal.

It may happen that because of additions to the ladle during a blow orfor other reasons, the flame intensity may drop prematurely and beforethe true bench mark point is reached, indicating the final stages of theblow. For this reason, the pulses on lead 52 are counted within thecomputer 44; and if the count indicates that not enough oxygen has beenblown onto the surface of the molten bath to justify a conclusion thatthe bench mark has been reached, then the computer program fordetermining the carbon remaining in the bath will not be initiated.

The computer 44 is provided with an input panel 58 which feeds signalsto the computer 44 indicating that the oxygen is ON, that the oxygen isOFF, that additions are being made to the vessel 10, and that the vesselis being tapped. The oxygen ON signal initiates the computer programwhile the oxygen OFF signal terminates the program. Furthermore, thevessel addition signal interrupts the computer program such that a falseindication of the bench mark 56 shown in FIG. 28 will not occur whenadditions are being made to the vessel. The vessel tap signal isnecessary to indicate the end of the current heat and permitsinitialization of all software for the subsequent heat.

Once the oxygen ON signal from the input panel 58 is applied-to thecomputer, a sampling circuit 60 samples the output of filter circuit 42,which is an analog signal, once each second. These samples are I thencompared in accordance with pattern recognition techniques in comparator62 with stored data in a storage bank 64 in the computer. When thesamples indicate that the bench mark has been reached, the comparatorproduces an output signal on lead 66 to initiate the program fordetermining the carbon content of the steel within the vessel 10. Notethat the pulses on lead 52, the number of which is proportional to totaloxygen flow into the vessel, are counted by counter 68. Until the countof counter 68 reaches a predetermined level, indicating that the blow isin the region where a bench mark can occur, a disabling signal isapplied to the comparator 62 such that it cannot initiate the carbonrcmovalprogram.

As was explained above, the carbon removal program initially determinesthe carbon remaining in the melt at the point of bench mark recognitionand thereafter calculates, once each second, the carbon removed duringthat second. The carbon removed during the time period is thensubtracted from the previously determined carbon content to derive theinstantaneous carbon content in the bath.

The initial carbon content C is determined in circuitry 70 within thecomputer in accordance with the equation:

C,,=k,+k,(FL/O C) 1 where:

C initial carbon content;

F the flame intensity at the point of bench mark recognition;

0 C the oxygen consumed at the point of recognition; and

k and k are empirically derived coefficients. Accordingly, it isnecessary to apply to the circuit 70 a signal on lead 72 representativeof the total oxygen consumed and a signal on lead 74 indicative of theflame intensity.

When the bench mark is recognized, it is also necessary to determine thecarbon removal rate, XRM, in accordance with the equation:

XRM=k (0 C) 2 where:

0 C is the oxygen consumed at the point of bench mark recognition; and

k is an empirically derived coefficient.

This computation is performed in circuitry represented by the referencenumeral 76 in FIG. 1 which has applied thereto the output of counter 68representing 0 C.

The constants k k and k in Equations (1) and (2) above are determinedfrom actual operating experience. The technique used is simple linearregression analysis which determines a least squares fit of a curve tothe data points. In determining k and k for example, the quantity Cdetermined by chemical analysis, is plotted against the ratio FL/O 2Cfor various samples. The resulting curve is defined by Equation (1)above and from this k and k can be determined. A similar technique isused to determine k Having thus determined the carbon content in thebath, C and the carbon removal rate, XRM, at the point of bench markrecognition, the computer then computes, once each second, thedifference in carbon content, AC,, representative of the difference incarbon content between the reading just taken and the preceding reading.This is performed in circuitry represented by the numeral 78 in FIG. 1in accordance with the equation:

The following table gives the ranges of constants k -k for Equations(1-(3) above:

where:

C, is the previous estimate of carbon level; FL is the current flameintensity as measured by device FL is the flame intensity associatedwith the point of bench mark recognition; O FR is the present oxygenflow rate as derived from lead 46 connected to flow meter 48; and k k,,and k are empirically derived coefficients. k.,, k and k are againdetermined from actual operating data using the mathematical techniqueof multiple regression analysis.

Having determined AC,, it is then necessary to determine C,, which isthe instantaneous carbon level in accordance with the equation:

i-l G (4) In order to do this, the value of C, is stored in a movingtable 82 during the next one-second sampling period. The previous valuefor C, becomes C and is fed back into the circuit 80 for computation ofa new value of C which, in turn, is fed back to the moving table 82.

Having determined C this can be displayed to an operator whereby he canterminate the oxygen flow when the indicated level of oxygen matchesthat required for a specific steel analysis. Alternatively, anelectrical signal proportional to desired carbon content can be comparedin comparator 84 with the calculated value of C, from circuit 80. Whenthe two are the same, a signal on lead 86 will cause oxygen flow controlmeans 88 to shut off valve 24 and, hence, terminate the flow of oxygento the bath.

Constant Low High k,, k and k are not actually constants but variableswhich are constant for a given blow only. Using feedback on actualperformance, these parameters are evaluated after each heat and longrange adaptive corrections made on actual performance. The manner inwhich these corrections are programmed into the computer is well withinthe skill of the art.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention.

I claim as my invention:

1. In the method for controlling the refining ofa molten bath of steelin a basic oxygen vessel, the steps of continually measuring during anoxygen blow the intensity of the flame at the mouth of the vessel,continually measuring the flow rate of oxygen into the vessel and thetotal flow of oxygen into the vessel, determining from a considerationof measured flame intensity dropoff a bench mark signaling that theoxygen blow is in its final stages, periodically electrically computingcarbon in the bath at the point of bench mark recognition from theequation:

C =k, +k (FL/0 C) where C is the carbon in the bath at the point ofbench mark recognition, k and k: are constants determined by linearregression analysis, FL is the value of the flame intensity at the pointof bench mark recognition and 0 C is the total oxygen consumed from thebeginning of refining to the point of bench mark recognition, thereafterperiodically computing from the point of bench mark recognition theamount of carbon left in the bath and stopping the oxygen blow when thecomputed amount of carbon left in the bath reaches a desired carbonlevel, and after each refining process computing the constants k and kbased upon the values of C and (FL/0 C) for the previous refiningprocess and using the new computed values for a succeeding refiningprocess.

2. The method of claim 1 including the step of determining the carbonremoval rate at the point of bench mark recognition from a considerationof the total oxygen flow into the vessel at the point of bench markrecognition.

3. The method of claim 2 wherein the carbon removal rate at the point ofbench mark recognition is determinedfrom the equation:

where XRM is the carbon removal rate at the point of bench markrecognition, k is a constant determined by linear regression analysisfrom actual operating experience and 0 C is the total oxygen consumedfrom the beginning of refining to the point of bench mark recognition.

where C is a previous value of predicted carbon as computed according tothe method of claim 6 and AC,-

is the difference between C,- and C 6. The method of claim 5 whereinAC,- is computed from the equation:

where k k and k are constants, FL is the flame intensity at the point ofbench mark recognition, FL is the current flame intensity, O FR is thecurrent oxygen flow rate and XRM is the carbon removal rate at the pointof bench mark recognition.

1. In the method for controlling the refining of a molten bath of steelin a basic oxygen vessel, the steps of continually measuring during anoxygen blow the intensity of the flame at the mouth of the vessel,continually measuring the flow rate of oxygen into the vessel and thetotal flow of oxygen into the vessel, determining from a considerationof measured flame intensity drop-off a bench mark signaling that theoxygen blow is in its final stages, periodically electrically computingcarbon in the bath at the point of bench mark recognition from theequation: Co k1 + k2 (FL/O2C) where Co is the carbon in the bath at thepoint of bench mark recognition, k1 and k2 are constants determined bylinear regression analysis, FL is the value of the flame intensity atthe point of bench mark recognition and O2C is the total oxygen consumedfrom the beginning of refining to the point of bench mark recognition,thereafter periodically computing from the point of bench markrecognition the amount of carbon left in the bath and stopping theoxygen blow when the computed amount of carbon left in the bath reachesa desired carbon level, and after each refining process computing theconstants k1 and k2 based upon the values of Co and (FL/O2C) for theprevious refining process and using the new computed values for asucceeding refining process.
 2. The method of claim 1 including the stepof determining the carbon removal rate at the point of bench markrecognition from a consideration of the total oxygen flow into thevessel at the point of bench mark recognition.
 3. The method of claim 2wherein the carbon removal rate at the point of bench mark recognitionis determined from the equation: :XRM k3 (O2C) where XRM is the carbonremoval rate at the point of bench mark recognition, k3 is a constantdetermined by linear regression analysis from actual operatingexperience and O2C is the total oxygen consumed from the beginning ofrefining to the point of bench mark recognition.
 4. The method of claim3 including the step of computing k3 after each refining process basedupon the values of XRM and (O2C) for the previous refining process andusing the new computed value of k3 for the next refining process.
 5. Themethod of claim 1 wherein the amount of carbon left in the bath at anypoint, Ci, is computed from the equation: :Ci Ci 1 - Delta Ci where Ci 1is a previous value of predicted carbon as computed according to themethod of claim 6 and Delta Ci is the difference between Ci and Ci 1.